1. Field of the Invention
The present invention relates to a process for producing a readily sinterable powder of perovskite or its solid solution. Such perovskite and solid solution are widely used in the field of functional ceramics such as piezo electric elements, optoelectronics materials, dielectrics, semiconductors, and sensors. Reflecting the recent progress in the high quality functional ceramics, there has been an increasing demand for an inexpensive, readily sinterable, uniform, high packing-density powder material which satisfies the requirements for such high quality functional ceramics.
2. Description of the Prior Art
A dry process and a wet process have been known for the production of a powder material of perovskite or its solid solution.
The dry process is a process wherein starting material powders are mixed and the mixture is then calcined. By such a process, however, it is hardly possible to obtain a powder material of a uniform composition. However, the product will have no adequate sinterability, and it will be difficult to prepare, from such a product, a perovskite solid solution having an excellent functionality.
On the other hand, the wet process is a process in which a solution containing all the constituting components, is prepared and added to a precipitating solution such as an alkaline solution for coprecipitation, and the coprecipitates thereby formed are then dried and calcined (hereinafter referred to as a "coprecipitation method").
According to this coprecipitation method, a powder having a superior uniformity is obtainable. However, because of this uniformity, particles are likely to coagulate during the precipitation, drying or calcining process and form secondary particles, whereby it is hardly possible to obtain a readily sinterable product.
Further, in the coprecipitation method, the concentration of the precipitating solution at the time of the addition is uniform, and the precipitate-forming abilities of the respective components differ from one another. For instance, a certain component may precipitate 100%, while another component may not precipitate completely, whereby it may be difficult to obtain a desired composition.
Furthermore, in many cases, the perovskite solid solution contains lead (a metal element coordinated with 12 oxygen atoms, which corresponds to component A mentioned hereinafter) and titanium (a metal element coordinated with 6 oxygen atoms, which corresponds to component B mentioned hereinafter) simultaneously. For the industrial production of such a solid solution, it is desired to use titanium tetrachloride or titanium sulfate which is inexpensive as a titanium material. However, when titanium tetrachloride, for example, is used for the coprecipitation method, chlorine ions in the titanium tetrachloride are likely to react with lead ions and form white precipitates. Therefore, titanium tetrachloride is hardly useful for the coprecipitation method. Such undesirable precipitation may be prevented by using titanium oxynitrate (TiO(NO.sub.3).sub.2) instead of titanium tetrachloride. However, it is not practical to use titanium oxynitrate in an industrial operation, since it is too expensive.